The iPad 3G may require a micro SIM—a lilliputian SIM card incompatible with anything else you own—but AT&T will be selling each bundled with a full SIM card adapter—meaning you could potentially use your data plan elsewhere. UPDATE:

UPDATE: Actually, a better look from BGR shows us that this seems to be a punch out scenario in which you can't reuse the larger SIM casing once you choose to break away the Micro SIM. It's probably just going to make stocking at AT&T easier. Luckily, given that SIM and Micro SIM have the same contacts, adapters (even homemade ones) are likely to be available soon. [BGR and Engadget]